1. Field of the Invention
This invention relates to a developer carrying member used when an electrostatic latent image formed on an electrostatic latent image bearing member such as an electrophotographic photosensitive member or an electrostatic recording dielectric material is rendered visible by development. It also relates to a developing assembly, an image forming apparatus and process cartridge using such a developer carrying member.
2. Description of the Related Art
As an example of developing assemblies for visualizing electrostatic latent images formed on a photosensitive drum serving as an electrostatic latent image bearing member by the use of a magnetic toner as a one component type developer, such an assembly as shown in FIG. 6 is known, in which a developer container 53 holds a magnetic toner 54 as the one component type developer, and electric charge having a reverse polarity to the electric charge of the electrostatic image formed on a photosensitive drum 51 and to the development standard potential is imparted to magnetic toner particles by the friction between particles of the magnetic toner and the friction between a developing sleeve 58 as a developer carrying member and the magnetic toner particles. The magnetic toner thus charged is very thinly coated on the developing sleeve 58 by means of a magnetic blade 52 and then transported to the developing zone, D, defined by the photosensitive drum 51 and the developing sleeve 58. In the developing zone D, the magnetic toner is attracted by the action of a magnetic field formed by a magnet 55 stationarily set on the developing sleeve 58 to visualize the electrostatic latent image on the photosensitive drum 51. In the drawing, letter symbols A and B denote the directions of rotation of the developing sleeve 58 and the photosenitive drum 51, respectively. Reference numeral 59 denotes a development bias means for applying a development bias voltage at the time of development; and 60, denotes an agitating element for agitating the magnetic toner 54 inside the developer container 53.
When, however, such a one component type developer is used, it is difficult to control the toner charging. Although various means have been considered in connection with developers, the problems concerning non-uniformity of charging and running stability of charging are not completely solved.
Especially when the developing sleeve is repeatedly rotated on, the toner coated on the developing sleeve comes to have a too large of a charge quantity due to contact with the developing sleeve, so that the toner and the developing sleeve surface attract each other on account of the reflective force and the toner turns immobile on the surface of the developing sleeve. This phenomenon is called "charge-up", which especially tends to occur in an environment of low humidity. Once such charge-up has occurred, the toner forming an upper layer on the sleeve is difficult to charge so that the quantity of toner participating in development is lowered, raising such problems that line images are made thinner or solid-image density is reduced.
Moreover, the toner layer may be formed in a different state at image areas (where toner is consumed) and non-image areas to have been charged in different conditions, and hence, for example, if the position where a solid image with a high image density has been once formed by development comes to the development position on the next rotation of the developing sleeve and a latent halftone image is developed at that position, a mark of the solid image may appear on the image. Such a phenomenon, which is called "sleeve ghost", is liable to occur.
Recently, in order to make electrophotographic image quality much higher, toners have been made to have smaller particle diameters and made finer. For example, in order to improve resolution and sharpness and faithfully reproduce latent images, it is common to use toners with a weight average particle diameter of about 6 to 9 .mu.m. Also, for the purpose of making a copying time shorter and power consumption smaller, there is a tendency toward lower fixing temperatures. Under such circumstances, the toner is more liable to electrostatically adhere onto the developing sleeve and when external physical force is applied, the surface of the developing sleeve may be contaminated and the toner may be melt-adhered.
As a method to prevent such phenomena, it is proposed to use in a developing assembly a developing sleeve having a metal substrate provided with a coat layer formed of a resin in which a solid lubricant and a conductive fine powder such as carbon powder are dispersed. The use of this method can be seen to greatly reduce the occurrence of the above phenomena. In this method, however, the surface shape of the developing sleeve is uneven and hence uniform charging still cannot be achieved sufficiently, leaving such problems on durability or running performance that the coat layer may become brittle.
Japanese Patent Application Laid-open No. 3-200986 discloses a developing sleeve comprising a metal substrate provided with a conductive coat layer formed of a resin in which a solid lubricant, a conductive fine powder such as carbon powder and also spherical particles are dispersed. In this developing sleeve, the shape of the developing sleeve surface is made uniform, the charging is uniform and wear resistance is improved. However, even in this developing sleeve, its running performance is sought to be more improved, e.g., in wear resistance of the conductive coat layer and prevention of toner contamination and melt-adhesion of toner when the sleeve has worn.